At first I used exceptions with saveOrFail() which worked, but I kept accidentally using save() which didn't. So I switched to enabling $throwValidationExceptions so that I'm always alerted when validation fails. I also had trouble with $rules and $messages so switched to using $rulesets and $validationMessages. Here is an example that works for me, hope this helps someone:

class User extends Eloquent implements UserInterface, RemindableInterface {
	
	use UserTrait, RemindableTrait, ValidatingTrait;
	
	protected $throwValidationExceptions = true;	// always throw validation exceptions
	
	/*protected $rules = [
		'username'	=> 'unique'
	];*/
	
	protected $rulesets = [
		'creating' => [
			'username'	=> 'required|unique'
		],
		'updating' => [
			'id'		=> 'required'
		],
		'deleting' => [
			'id'		=> 'required'
		]
	];
	
	protected $validationMessages = [
		'username.unique' => "That username is not available."
	];
	
	// ...
}